Design of experiments is a widely used approach to identification of the statistical properties of a system. The sequential probability ratio test (SPRT) is a specific sequential hypothesis test. It was developed by Abraham Wald. Sequential hypothesis testing is a form of statistical analysis wherein the sample size is not fixed in advance; rather, data is evaluated as it is collected, and further sampling is stopped in accordance with a pre-defined rule when significant results are observed.
Wald's suggestion was to take into account the results of the previous experiments, which resulted in the sequential testing. In many cases there is interest in comparison of two systems (for example, a new system and a basic system) in order to determine whether the new system behaves significantly better in a statistical sense.
Design of experiments finds its application, for example, in identification of chip yield (more generally, in manufacturing quality control), in the testing of human examinees, and statistical properties of individual tasks of processes, such as those of an enterprise. The standard approach to the problem of design of experiments for Wald's sequential tests is to run experiments with all different possible values of the test parameters in order to define characteristics of the test.